This invention relates to high voltage transformers.
A high voltage transformer (HVT), also referred to as a flyback transformer, is employed in television receivers to boost the B+voltage from the power supply to 20-30 kV applied to the cathode ray tube (CRT), as well as provide various secondary lower voltages for other circuits. The flyback transformer is driven by a pulse waveform of relatively high voltage, typically, between 1200 volts and 1400 volts.
The operating life of a winding in a flyback transformer can be increased by reducing voltage stress impressed on the flyback transformer when a high voltage is applied. Moreover, subjecting the winding to high voltages at higher frequencies increases adverse voltage stress effects on the windings.
A method for reducing voltage stress on a winding of a transformer divides the winding into multiple windings. For a primary winding, a full voltage is applied over multiple windings electrically coupled in series. Voltage impressed upon each of the individual windings is less than the sum total voltage impressed over all the windings. Similarly, for secondary windings the output voltage is developed over multiple windings so that voltage developed by an individual winding is less than the sum total voltage developed by all the secondary windings, which is the output voltage of the transformer.
A further consideration in the operation of a flyback transformer is losses in the inductive coupling between the primary and secondary windings. Magnetic losses occur when part of the primary winding overlaps an air gap in a ferrite core surrounded by the primary winding. That part of the winding overlapping with the air gap is not inductively coupled to the secondary winding, thereby reducing the amount of voltage at the primary side that can be stepped up to an output voltage level at the secondary side. The air gap in the ferrite core is critical to proper operation of the flyback transformer. The gap thickness in the ferrite core can be controlled by insertion of a plastic spacer between segments or ends of the ferrite core where an air gap is desired. Other transformer designs incorporate a core gap spacer integral with the primary bobbin.
Individually, the use of a core gap spacer or series coupled windings will not substantially increase the operating life of the winding and flyback transformer to a duration consistent with the life expectancy of equipment applications for the flyback transformer. Accordingly, there is a need for a high voltage transformer that satisfactorily reduces voltage related stress and magnetic losses on the transformer winding.
In accordance with an inventive arrangement there is provided a bobbin for primary windings in a high voltage transformer. The bobbin includes a sleeve; a barrier extending outwardly from the sleeve for spatially separating primary windings of a transformer wound around the sleeve that are electrically coupled to each other; and a spacer aligned with the barrier, extending inwardly from the sleeve and substantially defining an air gap for a ferrite core adapted to be mounted within the sleeve, the air gap being thereby substantially aligned with the barrier.
In accordance with a different inventive arrangement there is provided a transformer including a primary bobbin having a barrier extending outwardly therefrom and a spacer extending inwardly therefrom, the spacer being substantially aligned with the barrier; ferrite core segments each having first and second limbs, each of the first limbs having ends secured to each other, each of the second limbs adapted to be mounted within the primary bobbin and separated from each other by the spacer substantially defining an air gap between the second limbs, the air gap thereby being substantially aligned with the barrier; at least two primary windings coupled to each other, wound around the primary bobbin and separated by the barrier; a secondary bobbin around the primary windings and the primary bobbin; and a secondary winding wound around the secondary bobbin.
In accordance with a further different inventive arrangement there is provided a transformer including a primary bobbin with a radially and outwardly extended barrier, a ferrite core positioned within the primary bobbin, the ferrite core having an air gap and adapted to be mounted within the primary bobbin with the air gap substantially aligned with the barrier; at least two primary windings around the primary bobbin that are separated by the barrier and electrically coupled to each other; a secondary bobbin, and a secondary winding wound around the secondary bobbin and inductively coupled to the primary windings.